1. Field of the Invention
The present invention relates to a method of manufacturing a battery having a polygonal case and, more particularly, relates to a welding method of a polygonal battery case.
2. Description of the Related Art
Japanese Patent Laid Open No. 6-54660 discloses a battery having a rectangular metal case, which has a cover plate welded to an open end of the case by a laser beam welding device. The rectangular case is placed on a table, which moves a set time before the laser welding is started and continues to move for a while after the laser welding is completed in order to provide equal welding speed. A rectangular battery is, generally, effective to save the mounting space, and the laser beam welding provides good working efficiency without causing bad influence on the electrolyte and insulators of the battery as compared with other welding methods.
In general, since the rectangular battery case is formed from a metal plate to have corners with small curvature, the cover plate is also formed to have the corresponding corners with small curvature.
Some of the above described batteries, however, have problems of electric short circuiting, leaking of the electrolyte and other sealing problems.
The laser welding is usually carried out, as shown in FIG. 20, with a spot welding 201 along the interface line (hereinafter referred to as the weld interface) of the open end of the case 10 and outer edge of the cover 2 at a constant speed with regulated power. The straight line of the rectangular weld interface 200 is, therefore, melted to form a weld penetration area (portion melted by welding and thereafter becomes solid) having a V-shaped cross-section. On the other hand, the corners 202 of the rectangular weld interface 200 are melted more to have deeper V-shaped cross-section than the straight sides due to increase of the welding power per unit area, and the bottom of the V-shaped cross-section shifts to the inside due to difference in length between the outer periphery and the inner periphery of the weld bead 200 shown in FIG. 20, whereby a melted metal piece of the welded penetration area may get into the electrolyte contained inside the case and cause short circuiting, leakage of the electrolyte or breakdown of the battery.
In addition, when the cover 2 is put on the open end of the case 10 to be welded to the case 1 as shown in FIG. 21, the weld interface loses rigidity and the cover 2 may fall into the inside of the case 10 or pinholes may be formed in the weld interface.
If the case 1 is made of a thin metal plate, sags of the weld beads may be formed on a peripheral portion of the open end of the case 10 as shown in FIG. 22.